Contribution of Ionotropic Glutamatergic Receptors to Excitability and Attentional Signals in Macaque Frontal Eye Field

Abstract

Top-down attention, controlled by frontal cortical areas, is a key component of cognitive operations. How different neurotransmitters and neuromodulators flexibly change the cellular and network interactions with attention demands remains poorly understood. While acetylcholine and dopamine are critically involved, glutamatergic receptors have been proposed to play important roles. To understand their contribution to attentional signals, we investigated how ionotropic glutamatergic receptors in the frontal eye field (FEF) of male macaques contribute to neuronal excitability and attentional control signals in different cell types. Broad-spiking and narrow-spiking cells both required N-methyl-D-aspartic acid and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor activation for normal excitability, thereby affecting ongoing or stimulus-driven activity. However, attentional control signals were not dependent on either glutamatergic receptor type in broad- or narrow-spiking cells. A further subdivision of cell types into different functional types using cluster-analysis based on spike waveforms and spiking characteristics did not change the conclusions. This can be explained by a model where local blockade of specific ionotropic receptors is compensated by cell embedding in large-scale networks. It sets the glutamatergic system apart from the cholinergic system in FEF and demonstrates that a reduction in excitability is not sufficient to induce a reduction in attentional control signals.

Keywords: attention; frontal cortex; neuropharmacology; primate.

Figure 1

(A) Cartoon of the task. Monkeys fixated centrally. In total, 500 ms after fixation onset 3 colored gratings were presented equidistant from the fixation spot. One of the gratings was placed in the RF of the neuron under study. After a variable time (300–1400 ms), a central colored cue indicated which stimulus was behaviorally relevant on the current trial. The animal had to covertly monitor this stimulus and wait for it to change luminance (referred to as target dimming in the figure). The target dimming could occur first, second, or third in the sequence of dimming events (left to right in the figure). Distracter dimming had to be ignored by the monkey. Detection of target dimming was indicated by releasing a handheld touch bar. (For additional details see Methods). (B) Normalized spike average waveforms of all narrow-spiking (red) and all broad-spiking (blue) cells recorded and distribution of P2T times of recorded spike waveforms. Narrow-spiking P2T distribution is shown in red, broad-spiking P2T distribution is shown in blue. (C) Subdivision of different cell types into 5 different cell clusters. Dendrogram on the left shows the linkage between different cell clusters. Heat maps (next to the dendrograms) show the values of different neuronal features used for classification (x-axis) for all clustered cells (y-axis). Color coding is according to increasing standardized feature values (cluster variables). Dashed lines within heat maps show cell class borders along with cumulative cell numbers (i.e., cluster sizes can be inferred from these numbers). The color bar to the right of the standardized cluster-variable bar shows encoding of cell type along the narrow–broad-spiking divide (narrow: red, broad: blue). Distribution of P2T times for each cell cluster is shown on the right of each subplot. Red dashed line shows broad–narrow divide used in the current paper (240 μs).

Figure 2

Single-cell examples of glutamatergic modulation of firing rates and of attentional effects for broad (A,C) and narrow-spiking cells (B,D), when NMDA receptors (A,B) and when AMPA receptor (C,D) were blocked. Shown are raster plots and peristimulus time histograms for the 3 main task periods (stimulus onset, cue onset, time of the first dimming), separately for the 4 attention conditions (color coded). Attention to the RF, no drug condition: red; attention away from the RF, no drug condition: blue; attention to the RF, drug condition: green; attention away from the RF, drug condition: black.

Figure 3

Effect of attention and of drug application on population neuronal firing rates for broad- and narrow-spiking cells, when NMDA receptors and when AMPA receptors were blocked. Lines show mean, shaded area shows SEM. Red colors: attend RF, no drug applied; green colors: attend RF, drug applied; blue colors: attend away, no drug applied; black colors: attend away, drug applied.

Figure 4

Effect of glutamatergic blockers on neuronal excitability, quantified through drug modulation indices (DrugMI, across attention conditions). Data for broad-spiking cells are shown by black outlined histograms, those for narrow-spiking cells by gray-filled histograms. The different drug conditions are shown separately for all 3 task periods (poststimulus, postcue onset, before the dimming aligned response period). Dashed lines within the histograms indicate medians for narrow- (gray) and broad-spiking cells (black), dotted line shows zero location.

Figure 5

Attentional modulation quantified by calculating the AUROC when no drug (abscissa) was applied and when the drug of interest was applied (ordinate). (A) Black data points delineate AUROC values of narrow-spiking cells, gray data points those of broad-spiking cells. (B) Same as in (A), but for the 5 cell clusters identified by cluster analysis (B1–3, N1, 2).

Figure 6

Neuronal variability quantified by FFs and gain variance for the different drug and attention conditions for the 5 different cell clusters. (A) Individual FF data points (right) when NMDARs were blocked. Open symbols—no drug applied, filled symbols—drug applied. Mean FFs and standard deviation for the 5 different clusters are shown to the right. (B) Same as (A) but for the cells without and with AMPA receptors blocked. (C) mean ± SEM effect of attention on FFs across all cells. (D) Mean ± SEM effect of attention on FFs for the 5 different cell clusters. Table to the right of (D) shows significance of effects along with effect size (MM-ANOVA). (E) Comparison of FF mean ± SEM without and with drug applied. (F–J) same as (A–E), but for gain variance calculations.

Figure 7

Attentional modulation in a 2-stage network model. (A) Two different instantiations of attentional modulation in a 2-stage network model. Left raster plots and histograms show an example where attentional modulation was large (AUROC > 0.99). Right raster plots and histograms where attentional modulation was more modest (AUROC ~ 0.78). The upper histograms show the mean activity in the integration circuit. Blue colors show activity in the attend RF circuit, red colors the activity in the attend away circuit. Different color shadings indicate whether NMDA currents were unaffected (100% NMDA) or whether they were reduced (percentages indicate the amount of NMDA currents still available). The raster plots show activities of single cells from the circuit (from bottom to top 10 successive cells show the same amount of NMDA current available, i.e., NMDA drive increases every 10 cells). (B) Attentional modulation quantified as AUROC for 100 network runs (black dots) calculated between activity from 10 cells with 100% NMDA current drive versus 10 cells with reduced NMDA current drive (NMDA drive as percentage to maximum). The red symbol in the center indicates median AUROC for the 2 contrasted conditions. (C) Rate modulation histograms with reduced NMDA current drive, quantified as attentional modulation index (drug MI). Median drug MI is indicated by the thick dashed lines.